Polymer Compositions Having Improved Homogeneity and Odour, A Method For Making Them and Pipes Made Thereof

ABSTRACT

The present invention deals with polymer compositions suitable for making pipes. The compositions comprise a multimodal copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms wherein the multimodal ethylene copolymer has a density of from 924 to 960 kg/m 3 , a melt index MFR 5  of from 0.5 to 6.0 g/10 min, a melt index MFR 2  of from 0.1 to 2.0 g/10 min and a shear thinning index SHI 2.7/210  of from 2 to 50. The compositions further have a level of volatile compounds of at most 100 ppm by weight and/or a homogeneity rating of at most 3. In addition the multimodal copolymer comprises: 
     (A) from 35 to 60% by weight, based on the combined amount of components (A) and (B), of a low molecular weight ethylene polymer selected from ethylene homopolymer and a copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms and having a weight average molecular weight of from 5000 to 100000 g/mol and a density of from 945 to 975 kg/m 3 ; and
 
(B) from 40 to 65% by weight, based on the combined amount of components (A) and (B), of a high molecular weight copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms and having a weight average molecular weight of from 100000 to 1000000 g/mol and a density of from 890 to 935 kg/m 3 .

OBJECTIVE OF THE INVENTION

The present invention is directed for polymer compositions for making pipes. Especially, the present invention is directed for polymer compositions for making pipes having good mechanical properties, improved homogeneity, reduced level of volatiles and which are useful for transporting fluids under pressure. In addition the present invention is directed to pipes made of the polymer compositions and to methods of making them.

TECHNICAL BACKGROUND AND PRIOR ART

Pipes made of polyethylene have become popular in transporting water or gas, for instance in houses and in municipal water distribution. Polyethylenes having a high or medium density are frequently used in such pipes due to their good mechanical properties and ability to withstand pressure. Especially pipes made of multimodal polyethylene having a density of from about 947 to 953 kg/m³ have become increasingly popular. Such pipes and polymer compositions suitable for making them are disclosed, among others, in WO-A-00/01765, WO-A-00/22040, EP-A-739937, EP-A-1141118, EP-A-1041113, EP-A-1330490, EP-A-1328580 and EP-A-1425344. A co-pending European Patent Application No. 06020872.5 discloses flexible pressure-resistant pipes made of bimodal polyethylene and having a density of from 940 to 947 kg/m³.

Such pipes, however, suffer from the disadvantage that the pipes made of HDPE materials are not flexible enough so that they could be wound to a coil which is preferred in certain applications. Flexible pipes have been made from linear low density polyethylene and they are disclosed, among others, in EP-A-1574549. A co-pending European Patent Application No. 06024952.1 discloses flexible pipes in PE63 category having a density below 940 kg/m³. However, such pipes often lack the sufficient mechanical properties that are required from pipes used for transporting water or gas at high pressure. Especially such pipes do not qualify for PE80 or PE100 category.

SUMMARY OF THE INVENTION

The disadvantages of the prior art compositions and pipes are solved by the present polymer compositions and pipes made of them. Especially, the polymer compositions are flexible so that the pipes made of them can easily be bent and coiled. Additionally, the polymer compositions have a reduced level of volatile compounds which could cause bad odour. Subsequently the volatile compounds could migrate from the pipe into the water transported therein causing taste and/or odour problems in water. Furthermore, the polymer compositions have acceptable homogeneity combined with good mechanical properties and the resulting pipes fulfil the requirements of PE80 or PE100 classification without having an excessive amount of inhomogenities, such as white dots.

As seen from one aspect, the present invention provides polymer compositions comprising a multimodal copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms, the multimodal copolymer having a density from 924 to 960 kg/m³, an MFR₅ of from 0.4 to 6.0 g/10 min, preferably from 0.5 to 2.0 g/10 min, an SHI_(2.7/210) from 1 to 30, and the composition has a level of volatile compounds of at most 100 ppm by weight.

As seen from yet another aspect, the present invention provides pipes made of the polymer compositions comprising a multimodal copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms, the multimodal copolymer having a density from 924 to 960 kg/m³, an MFR₅ of from 0.4 to 6.0 g/10 min, preferably from 0.5 to 2.0 g/10 min, an SHI_(2.7/210) from 1 to 30, and the composition has a level of volatile compounds of at most 100 ppm by weight.

As seen from still another aspect, the present invention provides a method for making pipes wherein the method comprises the steps of:

(i) polymerising, in a first polymerisation step in a first polymerisation zone, in the presence of a single site polymerisation catalyst, ethylene, hydrogen and optionally one or more alpha-olefins having 4 to 10 carbon atoms to form the low molecular weight component (A) having a weight average molecular weight of from 5000 to 100000 g/mol and a density of from 945 to 977 kg/m³; (ii) polymerising, in a second polymerisation step in a second polymerisation zone, in the presence of a single site polymerisation catalyst, ethylene, one or more alpha-olefins having 4 to 10 carbon atoms and optionally hydrogen to form the high molecular weight component (B) having a weight average molecular weight of from 100000 to 1000000 g/mol and a density of from 890 to 935 kg/m³; wherein the first polymerisation step and the second polymerisation step may be conducted in any order and the subsequent step is conducted in the presence of the polymer produced in the prior step and the components (A) and (B) are present in the amounts of 30 to 70% and 70 to 30%, respectively, based on the combined amounts of components (A) and (B), and wherein the multimodal ethylene copolymer has a density from 924 to 960 kg/m³, an MFR₅ of from 0.4 to 6.0 g/10 min, preferably from 0.5 to 2.0 g/10 min, an SHI_(2.7/210) from 1 to 30, and the a composition comprising the multimodal ethylene copolymer has a level of volatile compounds of at most 100 ppm by weight.

As seen from a further aspect, the present invention provides the use of the composition comprising a multimodal copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms, the multimodal copolymer having a density from 924 to 960 kg/m³, an MFR₅ of from 0.4 to 6.0 g/10 min, preferably from 0.5 to 2.0 g/10 min, an SHI_(2.7/210) from 1 to 30, and a the composition has a level of volatile compounds of at most 100 ppm by weight, for making pipes.

DETAILED DESCRIPTION

Below the invention, its preferred embodiments and its advantages are described more in detail.

Multimodal Ethylene Polymer

The multimodal ethylene copolymer is a copolymer of ethylene and at least one alpha-olefin having from 4 to 10 carbon atoms. It has a density of from 924 to 960 kg/m³. Additionally it has a melt index MFR₅ of from 0.4 to 6.0 g/10 min, preferably from 0.5 to 2.0 g/10 min and more preferably from 0.6 to 1.4 g/10 min. Further, it typically has a melt index MFR₂ of from 0.1 to 2.0 g/10 min, preferably from 0.2 to 1.0 g/10 min and more preferably from 0.2 to 0.45 g/10 min. Additionally it has a shear thinning index SHI_(2.7/210) of from 1 to 30, preferably from 2 to 20 and more preferably from 3 to 15.

The multimodal ethylene copolymer preferably has a weight average molecular weight of from 75000 g/mol to 250000 g/mol, more preferably from 100000 g/mol to 250000 g/mol and in particular from 120000 g/mol to 220000 g/mol. Additionally, it preferably has a number average molecular weight of 15000 g/mol to 40000 g/mol, and more preferably 18000 to 30000 g/mol. It furthermore preferably has a ratio Mw/Mn of from 4 to 15, more preferably from 4 to 10.

Preferably the multimodal ethylene copolymer comprises a low molecular weight ethylene polymer component (A) and a high molecular weight ethylene copolymer component (B). Especially, the composition preferably contains from 30 to 70% the low molecular weight polymer (A) and more preferably from 35 to 50%. In addition, the composition preferably contains from 70 to 30% by weight of the copolymer (B) and more preferably from 65 to 50%. The percentage figures are based on the combined weight of components (A) and (B). The components (A) and (B) are explained more in detail below.

The low molecular weight polymer component (A) is an ethylene homopolymer or a copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms. It preferably has a weight average molecular weight Mw of from 5000 to 100000, more preferably from 10000 to 100000 g/mol, even more preferably from 15000 to 80000 g/mol and in particular from 15000 to 50000 g/mol. Preferably it has a melt index MFR₂ of from 20 to 1500 g/10 min. Moreover, it preferably has a narrow molecular weight distribution having a ratio of the weight average molecular weight to the number average molecular weight of from 2 to 5, more preferably from 2 to 4 and in particular from 2 to 3.5. Furthermore, it preferably has a density of from 945 to 977 kg/m³. Especially preferably the low molecular weight ethylene polymer (A) is an ethylene homopolymer.

The high molecular weight polymer component (B) is a copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms. It preferably has a weight average molecular weight Mw of from 100000 to 1000000 g/mol and more preferably from 150000 to 500000 g/mol. Preferably it has a melt index MFR₂ of from 0.01 to 0.3 g/10 min. Moreover, it preferably has a narrow molecular weight distribution having a ratio of the weight average molecular weight to the number average molecular weight of from 2 to 3.5. Furthermore, it preferably has a density of from 890 to 935 kg/m³, more preferably from 900 to 929 kg/m³.

By ethylene homopolymer is meant a polymer which substantially consists of ethylene units. As the process streams may have small amount of other polymeriseable species as impurities the homopolymer may contain a small amount of units other than ethylene. The content of such units should be lower than 0.2% by mole, preferably less than 0.1% by mole.

By copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms is meant a copolymer which has a majority of ethylene units and substantially consists of units derived from ethylene and alpha-olefins having from 4 to 10 carbon atoms. As the process streams may have small amount of other polymeriseable species as impurities the copolymer may contain a small amount of units other than ethylene and alpha-olefins having from 4 to 10 carbon atoms. The content of such units should be lower than 0.2% by mole, preferably less than 0.1% by mole.

The low molecular weight polymer component (A) and the high molecular weight polymer component (B) can also be blends of two or more different polymer fractions provided that each fraction, as well as the blend, meets the requirements given above for the specific component.

The multimodal ethylene copolymer may also contain minor amount of other polymer, such as prepolymer. The amount of such polymers should not exceed 5%, preferably not 2% by weight of the multimodal ethylene copolymer.

According to one embodiment of the invention the multimodal ethylene copolymer has a melt index MFR₅ of 0.5 to 2.0 g/10 min, preferably from 0.6 to 1.4 g/10 min. It has a density of from 925 to 935 kg/m³. Furthermore, it has a melt index MFR₂ of 0.1 to 1.0 g/10 min preferably from 0.2 to 0.45 g/10 min. It also has a shear thinning index SHI_(2.7/210) of from 1 to 30, preferably from 5 to 30.

According to another embodiment of the invention the multimodal ethylene copolymer has a melt index MFR₅ of 1.0 to 6.0 g/10 min, preferably from 1.4 to 6.0 g/10 min. It has a density of from 925 to 935 kg/m³. Furthermore, it has a melt index MFR₂ of 0.4 to 2.0 g/10 min, preferably from 0.5 to 2.0 g/10 min. It also has a shear thinning index SHI_(2.7/210) of from 2 to 30, preferably from 3 to 15.

Polymerisation Process

The multimodal ethylene copolymer is typically produced in a multistage polymerisation process in the presence of a single site catalyst.

In the multistage polymerisation process ethylene and alpha-olefins having from 4 to 10 carbon atoms are polymerised in a process comprising at least two polymerisation stages. Each polymerisation stage may be conducted in a separate reactor but they may also be conducted in at least two distinct polymerisation zones in one reactor. Preferably, the multistage polymerisation process is conducted in two cascaded polymerisation stages.

Catalyst

The polymerisation is typically conducted in the presence of a single site polymerisation catalyst. Preferably the single site catalyst is a metallocene catalyst. Such catalysts comprise a transition metal compound which contains a cyclopentadienyl, indenyl or fluorenyl ligand. Preferably the catalyst contains two cyclopentadienyl, indenyl or fluorenyl ligands, which may be bridged by a group preferably containing silicon and/or carbon atom(s). Further, the ligands may have substituents, such as alkyl groups, aryl groups, arylalkyl groups, alkylaryl groups, silyl groups, siloxy groups, alkoxy groups and like. Suitable metallocene compounds are known in the art and are disclosed, among others, in WO-A-97128170, WO-A-98/32776, WO-A-99/61489, WO-A-031010208, WO-A-031051934, WO-A-031051514, WO-A-20041085499, EP-A-1752462 and EP-A-1739103.

Especially, the metallocene compound must be capable of producing polyethylene having sufficiently high molecular weight. Especially it has been found that metallocene compounds having hafnium as the transition metal atom or metallocene compounds comprising an indenyl or tetrahydroindenyl type ligand often have the desired characteristics.

One example of suitable metallocene compounds is the group of metallocene compounds having zirconium, titanium or hafnium as the transition metal and one or more ligands having indenyl structure bearing a siloxy substituent, such as [ethylenebis(3,7-di(tri-isopropylsiloxy)inden-1-yl)]zirconium dichloride (both rac and meso), [ethylenebis(4,7-di(tri-isopropylsiloxy)inden-1-yl)]zirconium dichloride (both rac and meso), [ethylenebis(5-tert-butyldimethylsiloxy)inden-1-yl)]zirconium dichloride (both rac and meso), bis(5-tert-butyldimethylsiloxy)inden-1-yl)zirconium dichloride, [dimethylsilylenenebis(5-tert-butyldimethylsiloxy)inden-1-yl)]zirconium dichloride (both rac and meso), (N-tert-butylamido)(dimethyl)(η⁵-inden-4-yloxy)silanetitanium dichloride and [ethylenebis(2-(tert-butydimethylsiloxy)inden-1-yl)]zirconium dichloride (both rac and meso).

Another example is the group of metallocene compounds having hafnium as the transition metal atom and bearing a cyclopentadienyl type ligand, such as bis(n-butylcyclopentadienyl)hafnium dichloride, bis(n-butylcyclopentadienyl) dibenzylhafnium, dimethylsilylenenebis(n-butylcyclopentadienyl)hafnium dichloride (both rac and meso) and bis[1,2,4-tri(ethyl)cyclopentadienyl]hafnium dichloride.

Still another example is the group of metallocene compounds bearing a tetrahydroindenyl ligand such as bis(4,5,6,7-tetrahydroindenyl)zirconium dichloride, bis(4,5,6,7-tetrahydroindenyl)hafnium dichloride, ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride, dimethylsilylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride.

The single site catalyst typically also comprises an activator. Generally used activators are alumoxane compounds, such as methylalumoxane (MAO), tetraisobutylalumoxane (TIBAO) or hexaisobutylalumoxane (HIBAO). Also boron activators, such as those disclosed in US-A-2007/049711 may be used. The activators mentioned above may be used alone or they may be combined with, for instance, aluminium alkyls, such as triethylaluminium or tri-isobutylaluminium.

The catalyst may be supported. The support may be any particulate support, including inorganic oxide support, such as silica, alumina or titania, or polymeric support, such as polymer-comprising styrene or divinylbenzene. When a supported catalyst is used the catalyst needs to be prepared so that the activity of the catalyst does not suffer. Then the catalyst residues remaining in the product do not have negative impact on the taste and odour properties of the final polymer and the homogeneity of the polymer is not negatively affected.

The catalyst may also comprise the metallocene compound on solidified alumoxane or it may be a solid catalyst prepared according to emulsion solidification technology. Such catalysts are disclosed, among others, in EP-A-1539775 or WO-A-03/051934. It has surprisingly been found that when such catalyst is used the resulting multimodal polymer has improved homogeneity as indicated by reduced number and site of white dots so that the polymer composition has a low homogeneity rating according to ISO 18553 and improved taste and/or odour properties.

According to an especially preferred embodiment the catalyst comprises an organometallic compound of a transition metal of Group 3 to 10 of the Periodic Table (IUPAC), or of an actinide or lanthanide, in the form of solid catalyst particles and is prepared by a process comprising the following steps:

-   -   preparing a solution of one or more catalyst components;     -   dispersing said solution in a solvent immiscible therewith to         form an emulsion in which one or more catalyst components are         present in the droplets of the dispersed phase; and     -   solidifying said dispersed phase to convert said droplets to         solid particles and optionally recovering said particles to         obtain said catalyst.

Polymerisation

The multimodal ethylene copolymer may be produced in any suitable polymerisation process known in the art. Into the polymerisation zone is also introduced ethylene, optionally an inert diluent, and optionally hydrogen and/or comonomer. The low molecular weight ethylene polymer component is produced in a first polymerisation zone and the high molecular weight ethylene copolymer component is produced in a second polymerisation zone. The first polymerisation zone and the second polymerization zone may be connected in any order, i.e. the first polymerisation zone may precede the second polymerisation zone, or the second polymerisation zone may precede the first polymerisation zone or, alternatively, polymerisation zones may be connected in parallel. However, it is preferred to operate the polymerisation zones in cascaded mode. The polymerisation zones may operate in slurry, solution, or gas phase conditions or their combinations. Suitable reactor configurations are disclosed, among others, in WO-A-92112182, EP-A-369436, EP-A-503791, EP-A-881237 and WO-A-96/18662. Examples of processes where the polymerisation zones are arranged within one reactor system are disclosed in WO-A-99/03902, EP-A-782587 and EP-A-1633466.

It is often preferred to remove the reactants of the preceding polymerisation stage from the polymer before introducing it into the subsequent polymerisation stage. This is preferably done when transferring the polymer from one polymerisation stage to another. Suitable methods are disclosed, among others, in EP-A-1415999 and WO-A-00/26258.

The polymerisation in the polymerisation zone may be conducted in slurry. Then the polymer particles formed in the polymerisation, together with the catalyst fragmented and dispersed within the particles, are suspended in the fluid hydrocarbon. The slurry is agitated to enable the transfer of reactants from the fluid into the particles.

The polymerisation usually takes place in an inert diluent, typically a hydrocarbon diluent such as methane, ethane, propane, n-butane, isobutane, pentanes, hexanes, heptanes, octanes etc., or their mixtures. Preferably the diluent is a low-boiling hydrocarbon having from 1 to 4 carbon atoms or a mixture of such hydrocarbons. An especially preferred diluent is propane, possibly containing minor amount of methane, ethane and/or butane.

The ethylene content in the fluid phase of the slurry may be from 2 to about 50% by mole, preferably from about 3 to about 20% by mole and in particular from about 5 to about 15% by mole. The benefit of having a high ethylene concentration is that the productivity of the catalyst is increased but the drawback is that more ethylene then needs to be recycled than if the concentration was lower.

The temperature in the slurry polymerisation is typically from 50 to 115° C., preferably from 60 to 110° C. and in particular from 70 to 100° C. The pressure is from 1 to 150 bar, preferably from 10 to 100 bar.

The slurry polymerisation may be conducted in any known reactor used for slurry polymerisation. Such reactors include a continuous stirred tank reactor and a loop reactor. It is especially preferred to conduct the polymerisation in loop reactor. In such reactors the slurry is circulated with a high velocity along a closed pipe by using a circulation pump. Loop reactors are generally known in the art and examples are given, for instance, in U.S. Pat. No. 4,582,816, U.S. Pat. No. 3,405,109, U.S. Pat. No. 3,324,093, EP-A-479186 and U.S. Pat. No. 5,391,654.

It is sometimes advantageous to conduct the slurry polymerisation above the critical temperature and pressure of the fluid mixture. Such operation is described in U.S. Pat. No. 5,391,654.

In such operation the temperature is typically from 85 to 110° C., preferably from 90 to 105° C. and the pressure is from 40 to 150 bar, preferably from 50 to 100 bar.

The slurry may be withdrawn from the reactor either continuously or intermittently. A preferred way of intermittent withdrawal is the use of settling legs where slurry is allowed to concentrate before withdrawing a batch of the concentrated slurry from the reactor. The use of settling legs is disclosed, among others, in U.S. Pat. No. 3,374,211, U.S. Pat. No. 3,242,150 and EP-A-1310295. Continuous withdrawal is disclosed, among others, in EP-A-891990, EP-A-1415999, EP-A-1591460 and WO-A-2007/025640. The continuous withdrawal is advantageously combined with a suitable concentration method, as disclosed in EP-A-1310295 and EP-A-1591460.

If the low molecular weight ethylene polymer is produced in slurry polymerisation stage then hydrogen is added to the slurry reactor so that the molar ratio of hydrogen to ethylene in the reaction phase is from 0.1 to 1.0 mol/kmol, and preferably from 0.2 to 0.7 mol/kmol. Comonomer may then also be introduced into the slurry polymerisation stage so that the molar ratio of comonomer to ethylene in the reaction phase does not exceed 150 mol/kmol, and preferably not 50 mol/kmol. Especially preferably no comonomer is introduced into the slurry polymerisation stage.

If the high molecular weight ethylene polymer is produced in slurry polymerisation stage then hydrogen is added to the slurry reactor so that the molar ratio of hydrogen to ethylene in the reaction phase is at most 0.1 mol/kmol, preferably from 0.01 to 0.07 mol/kmol. Especially preferably, no hydrogen is introduced into the slurry polymerisation stage. Comonomer is introduced into the slurry polymerisation stage so that the molar ratio of comonomer to ethylene is from 50 to 200 mol/kmol, preferably from 70 to 120 mol/kmol.

The polymerisation may also be conducted in gas phase. In a fluidised bed gas phase reactor an olefin is polymerised in the presence of a polymerisation catalyst in an upwards moving gas stream. The reactor typically contains a fluidised bed comprising the growing polymer particles containing the active catalyst located above a fluidisation grid.

The polymer bed is fluidised with the help of the fluidisation gas comprising the olefin monomer, eventual comonomer(s), eventual chain growth controllers or chain transfer agents, such as hydrogen, and eventual inter gas. The fluidisation gas is introduced into an inlet chamber at the bottom of the reactor. To make sure that the gas flow is uniformly distributed over the cross-sectional surface area of the inlet chamber the inlet pipe may be equipped with a flow dividing element as known in the art, e.g. U.S. Pat. No. 4,933,149 and EP-A-684871.

From the inlet chamber the gas flow is passed upwards through a fluidisation grid into the fluidised bed. The purpose of the fluidisation grid is to divide the gas flow evenly through the cross-sectional area of the bed. Sometimes the fluidisation grid may be arranged to establish a gas stream to sweep along the reactor walls, as disclosed in WO-A-2005/087361. Other types of fluidisation grids are disclosed, among others, in U.S. Pat. No. 4,578,879, EP-A-600414 and EP-A-721798. An overview is given in Geldart and Bayens: The Design of Distributors for Gas-fluidized Beds, Powder Technology, Vol. 42, 1985.

The fluidisation gas passes through the fluidised bed. The superficial velocity of the fluidisation gas must be higher that minimum fluidisation velocity of the particles contained in the fluidised bed, as otherwise no fluidisation would occur. On the other hand, the velocity of the gas should be lower than the onset velocity of pneumatic transport, as otherwise the whole bed would be entrained with the fluidisation gas. The minimum fluidisation velocity and the onset velocity of pneumatic transport can be calculated when the particle characteristics are know by using common engineering practise. An overview is given, among others in Geldart: Gas Fluidization Technology, J. Wiley & Sons, 1986.

When the fluidisation gas is contacted with the bed containing the active catalyst the reactive components of the gas, such as monomers and chain transfer agents, react in the presence of the catalyst to produce the polymer product. At the same time the gas is heated by the reaction heat.

The unreacted fluidisation gas is removed from the top of the reactor and cooled in a heat exchanger to remove the heat of reaction. The gas is cooled to a temperature which is lower than that of the bed to prevent the bed from heating because of the reaction. It is possible to cool the gas to a temperature where a part of it condenses. When the liquid droplets enter the reaction zone they are vaporised. The vaporisation heat then contributes to the removal of the reaction heat. This kind of operation is called condensed mode and variations of it are disclosed, among others, in WO-A-20071025640, U.S. Pat. No. 4,543,399, EP-A-699213 and WO-A-94/25495. It is also possible to add condensing agents into the recycle gas stream, as disclosed in EP-A-696293. The condensing agents are non-polymerisable components, such as n-pentane, isopentane, n-butane or isobutane, which are at least partially condensed in the cooler.

The gas is then compressed and recycled into the inlet chamber of the reactor. Prior to the entry into the reactor fresh reactants are introduced into the fluidisation gas stream to compensate for the losses caused by the reaction and product withdrawal. It is generally known to analyse the composition of the fluidisation gas and introduce the gas components to keep the composition constant. The actual composition is determined by the desired properties of the product and the catalyst used in the polymerisation.

The catalyst may be introduced into the reactor in various ways, either continuously or intermittently. Among others, WO-A-01/05845 and EP-A-499759 disclose such methods. Where the gas phase reactor is a part of a reactor cascade the catalyst is usually dispersed within the polymer particles from the preceding polymerisation stage. The polymer particles may be introduced into the gas phase reactor as disclosed in EP-A-1415999 and WO-A-00/26258.

The polymeric product may be withdrawn from the gas phase reactor either continuously or intermittently. Combinations of these methods may also be used. Continuous withdrawal is disclosed, among others, in WO-A-00129452. Intermittent withdrawal is disclosed, among others, in U.S. Pat. No. 4,621,952, EP-A-188125, EP-A-250169 and EP-A-579426.

The top part of the gas phase reactor may include a so called disengagement zone. In such a zone the diameter of the reactor is increased to reduce the gas velocity and allow the particles that are carried from the bed with the fluidisation gas to settle back to the bed.

The bed level may be observed by different techniques known in the art. For instance, the pressure difference between the bottom of the reactor and a specific height of the bed may be recorded over the whole length of the reactor and the bed level may be calculated based on the pressure difference values. Such a calculation yields a time-averaged level. It is also possible to use ultrasonic sensors or radioactive sensors. With these methods instantaneous levels may be obtained, which of course may then be averaged over time to obtain time-averaged bed level.

Also antistatic agent(s) may be introduced into the gas phase reactor if needed. Suitable antistatic agents and methods to use them are disclosed, among others, in U.S. Pat. No. 5,026,795, U.S. Pat. No. 4,803,251, U.S. Pat. No. 4,532,311, U.S. Pat. No. 4,855,370 and EP-A-560035. They are usually polar compounds and include, among others, water, ketones, aldehydes and alcohols.

The reactor may also include a mechanical agitator to further facilitate mixing within the fluidised bed. An example of suitable agitator design is given in EP-A-707513.

If the low molecular weight ethylene polymer is produced in gas phase polymerisation stage then hydrogen is added to the gas phase reactor so that the molar ratio of hydrogen to ethylene is from 0.5 to 1.5 mol/kmol, and preferably from 0.7 to 1.3 mol/kmol. Comonomer may then also be introduced into the gas phase polymerisation stage so that the molar ratio of comonomer to ethylene does not exceed 20 mol/kmol, and preferably not 15 mol/kmol. Especially preferably no comonomer is introduced into the gas phase polymerisation stage.

If the high molecular weight ethylene polymer is produced in gas phase polymerisation stage then hydrogen is added to the gas phase reactor so that the molar ratio of hydrogen to ethylene is at most 0.4 mol/kmol, preferably at most 0.3 mol/kmol. Especially preferably, no hydrogen is introduced into the gas phase polymerisation stage. Comonomer is introduced into the gas phase polymerisation stage so that the molar ratio of comonomer to ethylene is from 5 to 50 mol/mol.

Powder Treatment

When the powder is withdrawn from the polymerisation section it is degassed and mixed with the desired additives. Degassing is preferably conducted by purging the polymer with gas at an elevated temperature.

One preferred method of purging the polymer is to pass a continuous stream of polymer powder through a vessel into which a gas stream is simultaneously passed. The gas stream may be either counter-current or co-current with the polymer stream, preferably counter-current. The residence time of the polymer is such a vessel may be from 10 minutes to 5 hours, preferably from about 30 minutes to about 2 hours. The gas used in purging may be ethylene, nitrogen, steam, air etc. Particularly good results have been obtained by using nitrogen as purging gas, which preferably contains a small amount of steam, such as from 100 ppm to 5% by weight, preferably from 100 ppm to 1%.

The temperature at which the polymer and gas are contacted may range from 30 to 100° C., preferably from 40 to 90° C. The temperature must be lower than the melting temperature of the multimodal ethylene copolymer. On the other hand, the temperature must be sufficiently high to make the volatile compounds to evaporate and migrate from the polymer into the gas stream.

Suitable gas stream in the method described above is from 0.01 to 5 ton gas per one ton of polymer.

Other suitable treatment methods may also be used. Thus, a batch of polymer may be purged in a vessel under a gas stream for a suitable period of time.

Polymer Composition

In addition to the multimodal ethylene copolymer the polymer composition comprises additives, fillers and adjuvants known in the art. It may also contain additional polymers, such as carrier polymers of the additive masterbatches. Preferably the polymer composition comprises at least 50% by weight of the multimodal ethylene copolymer, preferably from 80 to 100% by weight and more preferably from 85 to 100% by weight, based on the total weight of the composition.

Suitable antioxidants and stabilizers are, for instance, sterically hindered phenols, phosphates or phosphonites, sulphur containing antioxidants, alkyl radical scavengers, aromatic amines, hindered amine stabilizers and the blends containing compounds from two or more of the above-mentioned groups.

Examples of sterically hindered phenols are, among others, 2,6-di-tert-butyl-4-methyl phenol (sold, e.g., by Degussa under a trade name of Ionol CP), pentaerythrityl-tetrakis(3-(3′,5′-di-tert.butyl-4-hydroxyphenyl)-propionate (sold, e.g., by Ciba Specialty Chemicals under the trade name of Irganox 1010) octadecyl-3-3(3′5′-di-tert-butyl-4′-hydroxyphenyl)propionate (sold, e.g., by Ciba Specialty Chemicals under the trade name of Irganox 1076) and 2,5,7,8-tetramethyl-2(4′,8′,12′-trimethyltridecyl)chroman-6-ol (sold, e.g., by BASF under the trade name of Alpha-Tocopherol).

Examples of phosphates and phosphonites are tris (2,4-di-t-butylphenyl) phosphite (sold, e.g., by Ciba Specialty Chemicals under the trade name of Irgafos 168), tetrakis-(2,4-di-t-butylphenyl)-4,4′-biphenylen-di-phosphonite (sold, e.g., by Ciba Specialty Chemicals under the trade name of Irgafos P-EPQ) and tris-(nonylphenyl)phosphate (sold, e.g., by Dover Chemical under the trade name of Doverphos HiPure 4)

Examples of sulphur-containing antioxidants are dilaurylthiodipropionate (sold, e.g., by Ciba Specialty Chemicals under the trade name of Irganox PS 800), and distearylthiodipropionate (sold, e.g., by Chemtura under the trade name of Lowinox DSTDB).

Examples of nitrogen-containing antioxidants are 4,4′-bis(1,1′-dimethylbenzyl)diphenylamine (sold, e.g., by Chemtura under the trade name of Naugard 445), polymer of 2,2,4-trimethyl-1,2-dihydroquinoline (sold, e.g., by Chemtura under the trade name of Naugard EL-17), p-(p-toluene-sulfonylamido)-diphenylamine (sold, e.g., by Chemtura under the trade name of Naugard SA) and N,N′-diphenyl-p-phenylene-diamine (sold, e.g., by Chemtura under the trade name of Naugard J).

Commercially available blends of antioxidants and process stabilizers are also available, such as Irganox B225, Irganox 8215 and Irganox B561 marketed by Ciba-Geigy.

Suitable acid scavengers are, for instance, metal stearates, such as calcium stearate and zinc stearate. They are used in amounts generally known in the art, typically from 500 ppm to 10000 ppm and preferably from 500 to 5000 ppm.

Carbon black is a generally used pigment, which also acts as an UV-screener. Typically carbon black is used in an amount of from 0.5 to 5% by weight, preferably from 1.5 to 3.0% by weight. Preferably the carbon black is added as a masterbatch where it is premixed with a polymer, preferably high density polyethylene (HDPE), in a specific amount. Suitable masterbatches are, among others, HD4394, sold by Cabot Corporation, and PPM1805 by Poly Plast Muller. Also titanium oxide may be used as an UV-screener.

The polymer composition comprising the multimodal ethylene copolymer has preferably a low level of volatile compounds. Thus, the level of volatile compounds measured from the pellets made of the composition is at most 100 ppm by weight, preferably at most 75 ppm by weight and more preferably at most 50 ppm by weight. Typical values measured from the pelletised material may be from 1 to 30 ppm by weight.

Additionally the composition comprising the multimodal ethylene copolymer preferably has acceptable homogeneity. Thus, it preferably has a rating according to ISO 18553 of less than 6, more preferably of at most 5 and in particular of at most 4.5. As the person skilled in the art knows, the minimum rating is 0 for a completely homogeneous material.

Homogenisation and Pelletisation

The composition comprising the multimodal ethylene copolymer is homogenised and pelletised using a method known in the art. Preferably, a twin screw extruder is used. Such extruders are known in the art and they can be divided in co-rotating twin screw extruders, as disclosed in WO-A-98/15591, and counter-rotating twin screw extruders, as disclosed in EP-A-1600276. In the co-rotating twin screw extruder the screws rotate in the same direction whereas in the counter-rotating extruder they rotate in opposite directions. An overview is given, for example, in Rauwendaal: Polymer Extrusion (Hanser, 1986), chapters 10.3 to 10.5, pages 460 to 489. Especially preferably a counter-rotating twin screw extruder is used.

To ensure sufficient homogenisation of the polymer composition during the extrusion the specific energy input must be on a sufficiently high level. On the other hand, it must not be excessive, as otherwise degradation of polymer would occur. Also the additives could partly degrade due to too high energy input, and the degradation products of the polymer and the additives could cause offensive odour and/or taste in the polymer. The required SEI level depends somewhat on the screw configuration and design. Suitable levels of specific energy input (SEI) are from 200 to 300 kWh/ton, preferably from 210 to 290 kWh/ton. Especially good results have been obtained when the SEI is within the range disclosed above and a counter-rotating twin screw extruder having a screw design according to EP-A-1600276 is used.

Pipe and Pipe Manufacture

Pipes according to the present invention are produced according to the methods known in the art from the polymer composition as described above. Thus, according to one preferred method the polymer composition is extruded through an annular die to a desired internal diameter, after which the polymer composition is cooled.

The pipe extruder preferably operates at a relatively low temperature and therefore excessive heat build-up should be avoided. Extruders having a high length to diameter ratio L/D more than 15, preferably of at least 20 and in particular of at least 25 are preferred. The modern extruders typically have an L/D ratio of from about 30 to 35.

The polymer melt is extruded through an annular die, which may be arranged either as end-fed or side-fed configuration. The side-fed dies are often mounted with their axis parallel to that of the extruder, requiring a right-angle turn in the connection to the extruder. The advantage of side-fed dies is that the mandrel can be extended through the die and this allows, for instance, easy access for cooling water piping to the mandrel.

After the plastic melt leaves the die it is calibrated to the correct diameter. In one method the extrudate is directed into a metal tube (calibration sleeve). The inside of the extrudate is pressurised so that the plastic is pressed against the wall of the tube. The tube is cooled by using a jacket or by passing cold water over it.

According to another method a water-cooled extension is attached to the end of the die mandrel. The extension is thermally insulated from the die mandrel and is cooled by water circulated through the die mandrel. The extrudate is drawn over the mandrel which determines the shape of the pipe and holds it in shape during cooling. Cold water is flowed over the outside pipe surface for cooling.

According to still another method the extrudate leaving the die is directed into a tube having perforated section in the centre. A slight vacuum is drawn through the perforation to hold the pipe hold the pipe against the walls of the sizing chamber.

After the sizing the pipe is cooled, typically in a water bath having a length of about 5 metres or more.

The pipes according to the present invention preferably fulfil the requirements of PESO standard as defined in EN 12201 and EN 1555, evaluated according to ISO 9080, or alternatively ISO 4427.

The pipes according to the present invention are especially suited for transporting water or gas under pressure. Especially, they are suitable for transporting drinking water. No compounds producing offensive odour or taste into the water are migrated from the pipe into the water.

EXAMPLES Methods Melt Index

The melt flow rate (MFR) is determined according to ISO 1133 and is indicated in g/10 min. The MFR is an indication of the melt viscosity of the polymer. The MFR is determined at 190° C. for PE. The load under which the melt flow rate is determined is usually indicated as a subscript, for instance MFR₂ is measured under 2.16 kg load (condition D), MFR₅ is measured under 5 kg load (condition T) or MFR₂₁ is measured under 21.6 kg load (condition G).

The quantity FRR (flow rate ratio) is an indication of molecular weight distribution and denotes the ratio of flow rates at different loads. Thus, FRR_(21/2) denotes the value of MFR₂₁/MFR₂.

Density

Density of the polymer was measured according to ISO 1183/1872-2B.

For the purpose of this invention the density of the blend can be calculated from the densities of the components according to:

$\rho_{b} = {\sum\limits_{i}\; {w_{i} \cdot \rho_{i}}}$

where

-   -   ρ_(b) is the density of the blend,     -   w_(i) is the weight fraction of component “i” in the blend and     -   ρ_(i) is the density of the component “i”.

Molecular Weight

Mw, Mn and MWD are measured by Gel Permeation Chromatography (GPC) according to the following method:

The weight average molecular weight Mw and the molecular weight distribution (MWD=Mw/Mn wherein Mn is the number average molecular weight and Mw is the weight average molecular weight) is measured according to ISO 16014-4:2003 and ASTM D 6474-99. A Waters GPCV2000 instrument, equipped with refractive index detector and online viscosimeter was used with 2×GMHXL-HT and 1×G7000HXL-HT TSK-gel columns from Tosoh Bioscience and 1,2,4-trichlorobenzene (TCB, stabilized with 250 mg/L 2,6-Di tert-butyl-4-methyl-phenol) as solvent at 140° C. and at a constant flow rate of 1 mL/min. 209.5 μL of sample solution were injected per analysis. The column set was calibrated using universal calibration (according to ISO 16014-2:2003) with at least 15 narrow MWD polystyrene (PS) standards in the range of 1 kg/mol to 12 000 kg/mol. Mark Houwink constants were used as given in ASTM D 6474-99. All samples were prepared by dissolving 0.5-4.0 mg of polymer in 4 mL (at 140° C.) of stabilized TCB (same as mobile phase) and keeping for max. 3 hours at a maximum temperature of 160° C. with continuous gentle shaking prior sampling in into the GPC instrument.

As it is known in the art, the weight average molecular weight of a blend can be calculated if the molecular weights of its components are known according to:

${Mw}_{b} = {\sum\limits_{i}\; {w_{i} \cdot {Mw}_{i}}}$

where Mw_(b) is the weight average molecular weight of the blend, w_(i) is the weight fraction of component “i” in the blend and Mw_(i) is the weight average molecular weight of the component “i”.

The number average molecular weight can be calculated using the mixing rule:

$\frac{1}{{Mn}_{b}} = {\sum\limits_{i}\; \frac{w_{i}}{{Mn}_{i}}}$

where Mn_(b) is the weight average molecular weight of the blend, w_(i) is the weight fraction of component “i” in the blend and Mn_(i) is the weight average molecular weight of the component “i”.

Homogeneity

The homogeneity of the polymer samples containing carbon black was determined by using optical microscope according to the method ISO 18553 as follows.

A predetermined amount of the polymer of the Examples was mixed 5.75% of HE0880 carbon black masterbatch in a Brabender 350 E mixer with a Roller element at 190° C. temperature for 10 minutes. The screw speed was 20 RPM. Materials were then transferred to a compression moulding device for making about 3 mm thick plates (about 5×5 cm). Moulding conditions: 200 C during 10 minutes at low pressure and for 5 minutes at 114 bar and cooling at 15 C/min. Pellets of about 6 mm diameter were punched out from plates and then taken to homogeneity assessment.

A sample of the composition (containing the pigment) was obtained and at least 6 microtome cuts were made from different parts of the sample. Each cut had a thickness of about 12 μm (if carbon black was used; for other pigments the thickness may be from 15 to 35 μm). The diameter of the microtome cuts is from 3 to 5 mm. The cuts are evaluated at a magnification of 100. The diameters of the inhomogeneities (non-pigmented areas or “white spots”) are determined and a rating is given according to the rating scheme of ISO 18553. The lower is the rating the more homogeneous is the material.

Gel Level

The gels were determined from 0.3 mm sheet as follows.

A predetermined amount of the polymer of the Examples was mixed in a Brabender 350 E mixer with a Roller element at 190° C. temperature for 10 minutes. The screw speed was 20 RPM. Materials were then transferred to a compression moulding device for making about 0.3 mm thick sheets (about 20×20 cm). Moulding conditions: 200 C during 5 minutes at low pressure and for 5 minutes at 114 bar and cooling at 15 C/min.

The plates were inspected for gels over a glass table illuminated from below. The table is 0.5×0.3 m of size and equipped with three fluorescent lamps, each of them 15 W and with a warm white light. The lamps were covered with an opaque glass plate. The gels were divided into following classes according to size:

Class 1: More than or equal to 0.7 mm

Class 2: 0.4-0.7 mm Volatile Content

The total emission of the polymers was determined by using multiple head space extraction according to method as described below. If not mentioned otherwise all reported data refer to this method.

The method for measuring volatile components is carried out as follows:

The volatile components as described above were determined by using a gas chromatograph and a headspace method. The equipment was a Hewlett Packard gas chromatograph with a 25 m×0.32 mm×2.5 μm (length×diameter×size of packing material) non-polar column filled with DB-1 (100% dimethyl polysiloxane). A flame ionisation detector was used with hydrogen as a fuel gas. Helium at 10 psi was used as a carrier gas with a flow rate of 3 ml/min. After the injection of the sample the oven temperature was maintained at 50° C. for 3 minutes, after which it was increased at a rate of 12° C./min until it reached 200° C. Then the oven was maintained at that temperature for 4 minutes, after which the analysis was completed.

The calibration was carried out as follows: At least three and preferably from five to ten reference solutions were prepared, containing from 0.1 to 100 g of n-octane dissolved in 1 litre of dodecane. The concentration of octane in the reference solutions should be in the same area as the range of the volatiles in the samples to be analysed. 4 μl of each solution was injected into a 20 ml injection flask, which was thermostated to 120° C. and analysed. A calibration factor Rf for the area under the n-octane peak, A, vs. the amount of n-octane in the solution in μg, C, was thus obtained as Rf=C/A.

The analysis was conducted as follows: The polymer sample (about 2 grams) was placed in the 20 ml injection flask, which was thermostated to 120° C. and kept at that temperature for one hour. A gas sample from the injection flask was then injected into the GC. Before the analysis, a blind run was conducted, where an injection from an empty flask was made. The hydrocarbon emission E was then calculated as follows:

E=AT−Rf/W−1000000

wherein E is the hydrocarbon emission as μg volatile compounds per gram of sample, AT is the total area under the sample peaks in area counts, Rf is the calibration factor for n-octane in μg per area count, and W is the weight of the sample in grams.

Rheology

Rheological parameters such as Shear Thinning index SHI and Viscosity are determined by using a rheometer, preferably an Anton Pear Physica MCR 300 Rheometer on compression moulded samples under nitrogen atmosphere at 190° C. using 25 mm diameter plates and plate and plate geometry with a 1.8 mm gap according to ASTM 1440-95. The oscillatory shear experiments were done within the linear viscosity range of strain at frequencies from 0.05 to 300 rad/s (ISO 6721-1). Five measurement points per decade were made. The method is described in detail in WO 00/22040.

The values of storage modulus (G′), loss modulus (G″) complex modulus (G*) and complex viscosity (η*) were obtained as a function of frequency (ω). η₁₀₀ is used as abbreviation for the complex viscosity at the frequency of 100 rad/s.

Shear thinning index (SHI), which correlates with MWD and is independent of Mw, was calculated according to Heino (“Rheological characterization of polyethylene fractions” Heino, E. L., Lehtinen, A., Tanner J., Seppälä, J., Neste Oy, Porvoo, Finland, Theor. Appl. Rheol., Proc. Int. Congr. Rheol, 11th (1992), 1, 360-362, and “The influence of molecular structure on some rheological properties of polyethylene”, Heino, E. L., Borealis Polymers Oy, Porvoo, Finland, Annual Transactions of the Nordic Rheology Society, 1995.).

SHI value is obtained by calculating the complex viscosities at given values of complex modulus and calculating the ratio of the two viscosities. For example, using the values of complex modulus of 2.7 kPa and 210 kPa, then η*(2.7 kPa) and η*(210 kPa) are obtained at a constant value of complex modulus of 2.7 kPa and 210 kPa, respectively. The shear thinning index SHI_(2.7/210) is then defined as the ratio of the two viscosities η*(2.7 kPa) and η*(210 kPa), i.e. η(2.7)/η(210).

It is not always practical to measure the complex viscosity at a low value of the frequency directly. The value can be extrapolated by conducting the measurements down to the frequency of 0.126 rad/s, drawing the plot of complex viscosity vs. frequency in a logarithmic scale, drawing a best-fitting line through the five points corresponding to the lowest values of frequency and reading the viscosity value from this line.

Example 1 Preparation of the Catalyst Complex Preparation:

The catalyst complex used in the polymerisation example was bis(n-butyl cyclopentadienyl)hafnium dibenzyl, (n-BuCp)₂Hf(CH₂Ph)₂, and it was prepared according to “Catalyst Preparation Example 2” of WO 2005/002744, starting from bis(n-butylcyclopentadienyl)hafnium dichloride (supplied by Witco).

Activated Catalyst System:

The catalyst was prepared according to Example 4 of WO-A-03/051934, except that 98.4 mg of bis(n-butyl cyclopentadienyl)hafnium dibenzyl prepared as above was used as the metallocene compound instead of 80.3 mg bis(n-butyl cyclopentadienyl)hafnium dichloride.

Multi-Stage Polymerisation

A loop reactor having a volume of 50 dm³ was operated as a prepolymerisation reactor at 80° C. and 63 bar pressure. Into the reactor were introduced 50 kg/h of propane diluent, 2 kg/h ethylene, 1.8 g/h of hydrogen and 33 g/h of 1-butene. In addition, polymerisation catalyst prepared according to the description above was introduced into the reactor at a rate of 15 g/h.

The slurry was continuously withdrawn and directed into a subsequent loop reactor having a volume of 500 dm³, operated at 85° C. and 58 bar pressure. Into the reactor were additionally introduced 97 kg/h of propane diluent, 42 kg/h ethylene and 13 g/h of a gas mixture containing 25 vol-% of hydrogen in nitrogen. No additional comonomer was introduced into the reactor. The polymerisation rate was 34 kg/h and the conditions in the reactor as shown in Table 1.

The polymer slurry was withdrawn from the loop reactor and transferred into a flash vessel operated at 3 bar pressure and 70° C. temperature where the hydrocarbons were substantially removed from the polymer. The polymer was then introduced into a gas phase reactor operated at a temperature of 80° C. and a pressure of 20 bar. In addition 82 kg/h ethylene, 1.3 kg/h 1-hexene and 7 g/h hydrogen was introduced into the reactor. The conditions are shown in Table 1.

The resulting polymer was purged with nitrogen (about 50 kg/h) for one hour, stabilised with 3000 ppm of Irganox B225 and 1500 ppm Ca-stearate and then extruded to pellets in a counter-rotating twin screw extruder CIM90P (manufactured by Japan Steel Works) so that the throughput was 220 kg/h and the screw speed was 349 RPM.

Example 2

The procedure of Example 1 was repeated except that the operating conditions were slightly changed. The data is shown in Table 1.

Comparative Example

Into a 50 dm³ loop reactor operated at 60° C. temperature and 63 bar pressure as a prepolymerisation reactor were introduced ethylene (1.2 kg/h), propane diluent, hydrogen and a polymerisation catalyst. The solid catalyst component was a commercially available product produced and sold by Engelhard Corporation in Pasadena, USA under a trade name of Lynx 200 (now supplied by BASF). The solid component was used together with triethylaluminium cocatalyst so that the molar ratio of AIM was from 30 to 100. The resulting ethylene homopolymer had an MFR₅ of 3.5 g/10 min.

The slurry from the loop reactor was introduced into the second loop reactor having 500 dm³ volume operated at 85° C. and 57 bar where additional ethylene, propane and hydrogen were introduced. No comonomer was introduced into the loop reactor. The resulting slurry was withdrawn from the reactor into a flash vessel where the polymer was separated from the major fraction of the hydrocarbons at 70° C. and 3 bar. The polymer was directed into the gas phase reactor operated at 85° C. and 20 bar where additional ethylene, 1-butene comonomer and hydrogen were introduced. The final polymer was mixed with the additives and extruded. Data is shown in Table 1.

TABLE 1 Experimental conditions and data Example 1 2 C.E. Prepolymerisation reactor Ethylene feed, kg/h 2.0 2.0 N.D Butene feed, g/h 33 33 N.D Hydrogen feed, g/h 1.8 1.5 N.D Catalyst feed, g/h 15 15 N.D Loop reactor H₂/C₂, mol/kmol 0.18 0.17 947 Ethylene content, mol-% 11.9 12.7 5.6 Production rate, kg/h 33 33 22 Polymer MFR₂, g/10 min 11 13 442 Polymer Mw 68400 68000 Polymer density, kg/m³ 961 961 975 Gas phase reactor H₂/C₂, mol/kmol 0.10 0.08 48 C₄/C₂, mol/kmol 0.0 0.0 218 C₆/C₂, mol/kmol 3.2 2.4 0 Ethylene content, mol-% 54 57 18 Production rate, kg/h 35 37 28 Split, Prepol/LMW/HMW, %/%/% 3/48/49 3/48/49 2/44/54 Calculated density, kg/m³ 927 928 912 Extruder Throughput, kg/h 220 220 221 SEI, kWh/ton 265 270 280 Melt temperature, ° C. 225 230 226 Final polymer Polymer MFR₅, g/10 min 0.65 0.98 0.91 Polymer MFR₂₁, g/10 min 7.4 9.4 25 Polymer density, kg/m³ 944.8 944.3 941.0 SHI_(2.7/210) N.D. 7.6 33 η_(2.7), Pas N.D 28700 52000 Mw 181000 193000 N.D Mn 37500 38800 N.D Mw/Mn 4.8 5 N.D Volatiles, mg/kg 4 5 120 Homogeneity, ISO 18553 5.6 N.D. N.D. Gels in class 1, n 0 0 Gels in class 2, n 0 0 N.D = Not determined 

1. A polymer composition comprising a multimodal copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms, the multimodal copolymer having a density from 924 to 960 kg/m³, an MFR₅ of from 0.4 to 6.0 g/10 min, an SHI_(2.7/210) from 1 to 30, and the composition has a level of volatile compounds of at most 100 ppm by weight.
 2. The polymer composition according to claim 1 having a melt index MFR₅ of from 0.5 to 2.0 g/10 min, preferably from 0.6 to 1.4 g/10 min, and a melt index MFR₂ of from 0.1 to 2.0 g/10 min, preferably from 0.2 to 1.0 and in particular from 0.2 to 0.45 g/10 min.
 3. The polymer composition according to claim 1, wherein the multimodal copolymer is a copolymer of ethylene and one or more alpha-olefins having from 6 to 8 carbon atoms.
 4. The polymer composition according to claim 1, wherein the composition has homogeneity of less than 6 measured according to ISO
 18553. 5. The polymer composition according to claim 1, wherein the multimodal copolymer comprises: (A) from 30 to 70% by weight, based on the combined amount of components (A) and (B), of a low molecular weight ethylene polymer selected from ethylene homopolymer and a copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms and having a weight average molecular weight of from 5000 to 100000 g/mol and a density of from 945 to 977 kg/m³; and (B) from 30 to 70% by weight, based on the combined amount of components (A) and (B), of a high molecular weight copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms and having a weight average molecular weight of from 100000 to 1000000 g/mol and a density of from 890 to 935 kg/m³.
 6. The polymer composition according to claim 5, wherein the low molecular weight ethylene polymer (A) is an ethylene homopolymer.
 7. The polymer composition according to claim 5, wherein the high molecular weight copolymer is a copolymer of ethylene and one or more alpha-olefins having from 6 to 8 carbon atoms.
 8. A process for producing the polymer composition according to claim 5, comprising the steps of: (i) polymerising, in a first polymerisation step in a first polymerisation zone, in the presence of a single site polymerisation catalyst, ethylene, hydrogen and optionally one or more alpha-olefins having 4 to 10 carbon atoms to form the low molecular weight component (A) having a weight average molecular weight of from 5000 to 100000 g/mol and a density of from 945 to 977 kg/m³; (ii) polymerising, in a second polymerisation step in a second polymerisation zone, in the presence of a single site polymerisation catalyst, ethylene, one or more alpha-olefins having 4 to 10 carbon atoms and optionally hydrogen to form the high molecular weight component (B) having a weight average molecular weight of from 100000 to 1000000 g/mol and a density of from 890 to 935 kg/m³; wherein the first polymerisation step and the second polymerisation step may be conducted in any order and the subsequent step is conducted in the presence of the polymer produced in the prior step and the components (A) and (B) are present in the amounts of 30 to 70% and 70 to 30%, respectively, based on the combined amounts of components (A) and (B).
 9. The process according to claim 8, wherein said catalyst is a single site catalyst comprising a metallocene compound and an activator compound.
 10. The process according to claim 9, wherein the activator compound is an alumoxane, preferably a methylalumoxane.
 11. The process according to claim 9, wherein the metallocene compound is selected from the group consisting of [ethylenebis(3,7-di(tri-isopropylsiloxy)inden-1-yl)]zirconium dichloride, [ethylenebis(4,7-di(tri-isopropylsiloxy)inden-1-yl)]zirconium dichloride, [ethylenebis(5-tert-butyldimethylsiloxy)inden-1-yl)]zirconium dichloride, bis(5-tert-butyldimethylsiloxy)inden-1-yl)zirconium dichloride, [dimethylsilylenebis(5-tert-butyldimethylsiloxy)inden-1-yl)]zirconium dichloride, (N-tert-butylamido)(dimethyl)(η⁵-inden-4-yloxy)silanetitanium dichloride, [ethylenebis(2-(tert-butydimethylsiloxy)inden-1-yl)]zirconium dichloride, bis(n-butylcyclopentadienyl)hafnium dichloride, bis(n-butylcyclopentadienyl)dibenzylhafnium, dimethylsilylenebis(n-butylcyclopentadienyl)hafnium dichloride, bis[1,2,4-tri(ethyl)cyclopentadienyl]hafnium dichloride, bis(4,5,6,7-tetrahydroindenyl)zirconium dichloride, bis(4,5,6,7-tetrahydroindenyl)hafnium dichloride, ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride and dimethylsilylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride.
 12. A pipe comprising the composition according to claim
 1. 13. A pipe according to claim 12 comprising from 85 to 100% of said polymer composition.
 14. (canceled)
 15. (canceled) 